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10.8 Determining the Function of the Reagent
The last three sections help you become competent in performing all three steps.
Step 1 is determining the function of the reagent.
The function of the reagent is the main difference between substitution and elimination.
An elimination reaction occurs when the reagent functions as a base.
The first step in any specific case is to determine whether the reagent is a strong or weak base.
Students assume that a strong base must also be a strong nucleophile, but this is not always the case.
It is possible for a reagent to be weak.
It is possible for a reagent to be strong and weak at the same time.
Basicity and nucleophilicity do not always match up.
Let's compare H N- and HO-.
When we saw the factors determining charge stability in Chapter 3, we realized that oxygen is more stable than nitrogen.
Let's compare the two.
In Chapter 3, we saw that sulfur, being larger than oxygen, can be better at stabilizing a charge than oxygen can be.
HO- will be a stronger base because it will be more stable.
HO- is a better nucleophile than HS-.
Basicity and nucleophilicity are different concepts.
Basicity and nucleophilicity are measures of how stable the charge is.
An interesting effect comes into play when you have a large atom.
The electron density within the sulfur atom can move around as the sulfur atom approaches a compound with d+).
The rate of attack is very fast because of this effect.
The same is true for most of the halides.
The halides are too weak to be bases.
You don't need to worry about elimination reactions when you see one of these nucleophiles.
It is helpful to know that you don't need to worry about elimination reactions when you see a halide as a reagent.
Let's look at each category quickly.
The first category has reagents that only function as nucleophiles.
They are strong nucleophiles, but they are weak bases.
You should only look at substitution reactions when you see a reagent from this category.
This category does not include sulfuric acid.
The sulfur atom in sulfuric acid doesn't have a single pair, so it can't function as a nucleophile.
The name implies that sulfuric acid only works as an acid, so it is not listed in any of the other categories.
The second category contains only bases and not nucleophiles.
The hydride ion is the first reagent on the list.
Despite the presence of a negative charge, the hydride ion of NaH is not a good nucleophilic.
The hydride ion is a strong base.
The use of a hydride ion as a reagent shows that elimination will occur rather than substitution.
The second and third categories have butoxide in them.
It's a strong base and a strong nucleophile, so it's in the third category.
The sterically hindered butoxide prevents it from functioning as a nucleophile in most cases.
It is often used as a base to favor E2 over S 2.
There are reagents in the third category that are strong.
These reagents are used for bimolecular processes.
There are reagents in the fourth and final category that are weak.
The first step in predicting the products of a reaction is determining the nature and identity of the reagent.
To determine the category to which it belongs, you must analyze the reagent.
Let's practice with this skill.
The function of each of the reagents can be identified.
There are three main steps for predicting substitution and elimination reactions.
The first step is determining the function of the reagent.
The second step of the process in which we analyze the substrate and identify which mechanism operates is explored in this section.
There are four different categories of reagents.
The expected outcome must be explored with a primary, secondary, or tertiary substrate.
The flow chart summarizes all of the relevant information.
It is important to know the flow chart, but not to memorize it.
Understanding the reasons for all of these outcomes is more important.
A proper understanding on an exam will prove to be more useful than a set of rules.
The mechanism that operates is determined by the substrate.
For primary, secondary, and tertiary substrates, buOK can occur.
Bimolecular reactions are preferred when the reagent is strong and the base is strong.
The reactions are too slow.
Unimolecular reactions are favored under these conditions.
A tertiary alcohol will have an E1 reaction when treated with acid and heat.
The flow chart can be used to figure out which mechanism operates in a specific case.
Identifying the function of the reagent is the first step.
The next step is to identify the material.
Secondary substrates give N to steric hindrance.
Each of the following cases is expected to have a mechanism.
Don't worry about drawing the products yet.
In the next section, we will do that.
Pick out which mechanisms are operating.
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